1. Field of the Invention
This invention relates to a shock absorbing device for use in an installation of an integrated deck to fixed jacket legs.
2. Description of Related Arts
An insatiable demand for the ever depleting oil and gas has forced the oil companies to move their offshore production to deeper and more distant fields. Locations of offshore production activities have moved from its initial water depth of 20 meters to deeper waters of greater than 300 meters during the past decades. As such, problems related to construction and fabrication of production structures intensify and become more complex. This will also inevitably lead to increase in the construction cost of the oil platform and the danger of the hazardous offshore working environment. It is therefore more economical, safe and effective to undertake all the platform construction work onshore. When completed, the platform will be transported on a barge to the chosen offshore location. It is during the installation of the platform to the jacket legs, which is pre-installed at the offshore location, that a protective shock absorbing means is needed to prevent a possible damage to the platform due to the impact of the load transferred to the jacket legs. In view of this, various designs of the protective shock absorbing components have been effectively deployed in many offshore projects.
U.S. Pat. No. 4,222,683 disclosed a joint between a platform leg and a jacket leg. When the platform leg impacts on the jacket leg of the substructure, the impacts are damped in horizontal and vertical direction. The top of support leg is provided with a receiving funnel and a counter bearing for force-lockingly joining the platform legs and jacket legs. A gap remains free between the side walls of the platform legs and the receiving funnel. The said gap is filled with blocks of elastic material circumferentially spaced from one another for absorbing the impact of the downward platform leg. A metal slide rail is coupled to each of the blocks of the elastic material, without which the platform legs would be too strongly braked by the blocks of elastic material during downward motion.
U.S. Pat. No. 5,219,451 disclosed a system for transferring an integrated deck to an offshore jacket legs wherein the integrated deck has a number of depending legs for mating with the jacket legs. A stabbing tip assembly is provided for the depending legs to locate the depending legs to the associated jacket legs, absorb the vertical and lateral shock loads, and carry out the final transfer of weights of the integrated deck to the jacket legs. The stabbing tip assembly is equipped with a plurality of spaced apart elastomeric member, disposed in between an outer sleeve member and an inner sleeve member to absorb vertical and lateral shock loads occurring between the jacket legs and depending legs.
U.S. Pat. No. 4,848,967 disclosed a load transfer system for mating an integrated deck onto offshore jacket legs. A shock load absorbing means is used in the system, coupled to the lower end of the deck legs. The shock load absorbing means includes a plurality of annular elastomeric discs retained within a central tube, wherein the annular elastomeric discs are subjected to compression of a hydraulic cylinder from below. The hydraulic cylinder, in turn, is connected to a piston to be received by a receptacle on the jacket legs.
The elastic or elastomeric components of the shock absorbing means in the prior arts may suffer a shortcoming of having low stiffness, and hence a low withstandable load capacity, such that the shock absorbing means is prone to failure when a higher load is applied. Besides that, the elastomeric material having low stiffness tends to take a large amount of deflection which is not desirable for mating the integrated deck to the jacket legs. This will impose a constraint to the design of the oil platform, as the recent trend in design of oil platform construction is towards a higher loading capacity which involves an increase in the weight of the structural components and consequently an increase in total weight. Furthermore, for an optimum platform design, the diameter of the jacket leg may not be enlarged and hence a mere increase in the size of the shock absorbing device would not be a feasible solution to the higher loading capacity of the oil platform. Therefore, a high load capacity shock absorbing device having an optimum force/deflection curve is needed to tend to the higher oil platform weight of contemporary design.